This invention relates to wireless devices, and more particularly, to an antenna attached to a remote sensing apparatus or interrogator for use in detecting wireless communication devices such as radio frequency identification (RFID) tags or transponders affixed to livestock for identification of an animal as it passes proximate to the antenna.
Wireless memory devices for storing and transmitting data such as RFID transponders or tags are generally known in the art. Such transponders are typically used in the livestock for industry for tracking and accounting for livestock. During many stages of livestock management, it is necessary to record data about a particular animal, and to distinguish between animals. The RFID tags are useful not only for livestock, but in connection with other animals such as salmon and other migratory animals which are observed for various scientific and environmental purposes.
While RFID tags have certain advantages, particularly with respect to the amount or type of data that might be retrieved about a particular animal, there are also many disadvantages associated with the use of RFID tags. First, the transponder to be monitored is attached to the animal in a manner where the final position or orientation of the transponder cannot be controlled as it passes through the interrogation field in order to read the tag. In order to optimize reading of a transponder, the transponder antenna should be aligned with the various fields generated by the interrogator antenna. Poor environmental conditions can also affect the ability of an interrogator antenna to read the tag as it passes through the interrogation field. Yet another problem associated with RFID tags is an antenna that does not produce an adequate electromagnetic field, that is, a field that is not of sufficient strength to excite the tag in order that it may transmit its information. Yet another problem, which has arisen with respect to RFID tags is RF noise that is present in many industrial locations in which the tags are used. The RF noise can disrupt the interrogation field produced by the interrogator antenna thus preventing the ability to successfully interrogate a particular tag. As well understood by those skilled in the art, RF noise can be generated by surrounding electromagnetic devices that could be in close proximity to the interrogator antenna, or could be some distance away but the RF noise created by the remote electromagnetic device is of such sufficient strength that it still disrupts the interrogation field.
In the design of a great majority of antennas, it is undesirable to create null zones or out of phase areas within the electromagnetic field generated by the interrogator antenna. Such null zones represent areas in which an RFID tag cannot be interrogated because there is no electromagnetic energy within the null zone to excite the coil of the RFID tag. Therefore, creating null zones by an interrogator antenna is typically undesirable. However, out of phase areas within an electromagnetic field are also effective in canceling RF noise.
Therefore, it would be advantageous for an interrogator antenna to be developed which could alleviate RF noise by creating out of phase areas within the electromagnetic field generated by the antenna but at the same time, the antenna being capable of generating an omni-directional electromagnetic field that is still effective in interrogating an RFID transponder.
The present invention addresses the problems of RF noise by providing an antenna design which is effective in canceling RF noise, yet still is able to generate an omni-directional electromagnetic field for effective interrogation of RFID transponders.
In the preferred embodiment of the invention, the antenna comprises a pair of antenna coils that are placed in a crossing relationship with one another. More specifically, a first antenna coil is provided in a closed loop configuration which extends along a first axis, and a second antenna coil is provided of a closed loop arrangement which extends along a second axis, the second antenna coil being placed perpendicular to and in close proximity with the first antenna coil. The antenna coils are wired in parallel, in phase. The antennas should be oriented out-of-phase to create the required fields to achieve maximum performance. The crossing pattern formed by the pair of coils forms four external legs of the antenna. Electromagnetic fields are generated from each of the legs of the antenna in multiple directions away from the antenna thus providing a combined electromagnetic field which is able to interrogate an RFID transponder despite its potential for being oriented in many different angles as it passes through the electromagnetic field. Because of the directionally oriented magnetic fields produced by each of the legs of the antenna, five null zones or out of phase areas are generated within the crossing pattern of the antenna itself. There is no appreciable degradation of the electromagnetic field used for interrogation, because a tag to be interrogated always passes externally of the antenna and not actually through the plane of the antenna. In other words, the omni-directional field generated by the antenna has field portions that surround or isolate the null zones such that a moving transponder cannot reach a null zone without also contacting some portion of the electromagnetic field.
The size and power of the antenna can be adapted for the particular application in which the RFID transponders are to be interrogated. For livestock operations, one acceptable size for the antenna would include the crossing antenna coils each having a length of approximately 20 inches long and six inches wide, the width being measured between opposite sides of the closed loop coil.
In use, the antenna may be mounted to a walk through corridor or chute through which the livestock are directed. Cattle chutes or corridors are common structures found in most feedlocks or other cattle containment areas. Alternatively, the antenna of the invention could be mounted to any other stationary object, which is placed in close proximity to the RFID transponders to be interrogated.
The above described features and advantages of the invention will become more apparent from a review of the following description, taken in conjunction with the accompanying drawings.